Investigations on Neutronic Decoupling Phenomenon in Large Nuclear Reactors

Characteristic size (size of core expressed in terms of neutron migration length) of a nuclear reactor has been used as a simple thumb rule to assess the degree of neutronic coupling the reactor core. In the present paper, neutronic decoupling phenomena of reactor cores is studied using a sophisticated technique, called eigenvalue separation (EVS). A large eigenvalue separation reflects a distant higher harmonic and ensures neutronic coupling in respective neutron flux mode of the core. The EVS analysis technique has been used to optimize core geometry of pressurized water reactors, i.e., Height to Diameter (H/D) ratio of the reactor has been optimized to minimize the deep influence of higher harmonics on the stability. The study brings out that a typical H/D rat io beyond 1.3 should be avoided in large sized cores to maximize core neutronic coupling and hence the stability of the reactor.